When a vehicle travels on a road, wheel vibrations in reaction to the road are transmitted via the suspension to the vehicle body, and are excited by acoustic resonant properties of a closed space such as the vehicle passenger compartment, thereby generating road noise (muffled sounds referred to as “drumming noise”) having a peak at about 40 [Hz], and a bandwidth ranging from 20 to 150 [Hz]. An active noise control apparatus has been proposed for canceling out such road noise with canceling sounds which are in opposite phase to the road noise at an evaluation point (listening point) where a microphone is positioned (see Japanese Laid-Open Patent Publication No. 2007-025527).
The active noise control apparatus has an adaptive notch filter acting as a noise canceller (see “ADAPTIVE SIGNAL PROCESSING” by Bernard Widrow, Stanford University, Samuel D. Stearns, Sandia National Laboratories, 1985, Prentice-Hall, Inc., Englewood Cliffs, N.J. 07632 (FIG. 12.6, Page 317)). The active noise control apparatus generates a control signal dependent on the canceling sounds by having the adaptive notch filter function as a notch filter, which has a prescribed central frequency (road noise frequency) and passband.